The other intriguing question concerns why Russian President Vladimir Putin allowed the Assad regime to carry out the attack in the first place.
The Russians have detailed knowledge of the regime's chemical weapons stockpiles because the Russian military has spent the past 30 years helping the Syrians to develop their own weapons of mass destruction.
Moreover, it is the Russians that keep the Syrian warplanes flying as a result of the regular supplies of aviation fuel that Moscow regularly delivers to Damascus.
But Putin has nevertheless taken a big gamble by supporting the Syrian regime's attack on Idlib.
On Monday he received a personal telephone call from US President Donald Trump, who offered his commiserations for the terrorist attack on the St Petersburg underground.
As Trump has frequently indicated he wants to improve relations with the Kremlin, this presented Putin with a good opportunity to improve on his fraught relationship with the White House.
Instead, the Russian leader has opted to support the Assad regime's decision to intensify hostilities in Syria, which is no doubt Putin's way of testing the Trump Administration's mettle, to see whether it has the resolve to deal with countries accused of committing war crimes.
Mr Putin may well find that, this time, he has overplayed his hand.
Trump may have given out conflicting signals about the type of relationship he would like with the Kremlin.
However, hardened Cold War veterans such as Defence Secretary James Mattis and National Security Advisor General HR McMaster are unlikely to be taken in by the Russian leader's duplicity.